Serotonin in the brain influences everything from mood to fatigue to hunger to memory, and its levels are so precariously balanced that regulating it requires vast spirals of DNA. While outside factors like diet, sleep, and stress cause serotonin levels to fluctuate, damage at the source has far worse consequences: a single serotonin-related genetic defect can double or triple the likelihood of depression, obesity, or suicide.
Dysfunction in a gene called MAOA, the so-called "warrior gene," even goes so far as to cause psychopathy. The gene inhibits serotonin reception in the impulse-control region of the brain. Combined with exposure to abuse or trauma, studies show that the defect makes people highly susceptible to violent crime. Because the MAOA gene gets passed down only on the X sex chromosome of mothers, more men than women are psychopaths. For a man, only one X chromosome is inherited - the one from his mother, so, lacking an alternative, it gets expressed. A woman on the other hand gets an X from each parent, so that the normal MAOA gene usually inherited from her father can overrule a warrior one from her mother.
A neuroscientist named Jim Fallon of UC Irvine was one of the central figures involved in finding the correlation between the warrior gene and psychopathy. Psychopaths have fascinated Fallon for the past two decades, and perhaps, without knowing it, this was why: Fallon recently learned that no less than seven killers, including the famous Lizzie Borden, decorate his family tree. He subsequently analyzed the DNA and scanned the brains of every living member of his family. All their brain scans were normal (the orbital cortex of psychopaths shows little to no activity), and they all had normal MAOA genes - except one person. Fallon himself has all the trappings of a psychopath. In an interview for npr.org, he said, "I have the pattern, the risky pattern. In a sense, I'm a born killer."
Fallon describes his upbringing as "terrific," and credits it with stopping him from becoming a psychopath. Of course, this frightening insight into his own character, this realization of what he could have turned into if not for the love and kindness of a doting family, has given Fallon a degree of compassion for those genetically like him who didn't have it so easy. Most of us know to be conscious of the abuse and strife that violent criminals may have endured as children. But a setback as concrete as a genetic predisposition - a defect - is somehow even more deserving of compassion. In the words of Barbara Hagerty for NPR, "Enter the new world of 'neurolaw,' in which neuroscience is used as evidence in the courtroom."